High duty cycle ion spectrometer

ABSTRACT

An ion spectrometer is provided, comprising: an ion source, arranged to generate ions continuously with a first range of mass to charge ratios; and an ion trap, arranged to receive ions from the ion source along an axis, and to eject ions with a second range of mass to charge ratios orthogonally to that axis, the second range of mass to charge ratios being narrower than the first range of mass to charge ratios. In some embodiments, ions generated by the ion source continuously flow into the ion trap. Additionally or alternatively, ion optics receive ions ejected from the ion trap and cool the ions without substantial fragmentation. An ion analyser receives ions ejected from the ion trap or ion optics and separates the ions in accordance with at least one characteristic of the ions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. §120 and claims thepriority benefit of co-pending U.S. patent application Ser. No.14/746,753 filed Jun. 22, 2015, which is a continuation of U.S. patentapplication Ser. No. 14/360,345 filed May 23, 2014, now U.S. Pat. No.9,064,679, which is the United States National Stage Application, under35 USC 371, of International Application No. PCT/EP2012/073640 having aninternational filing date of Nov. 26, 2012 and designating the UnitedStates, which claims priority to GB 1120307.2, filed Nov. 24, 2011, saidapplications incorporated by reference herein in their entireties.

TECHNICAL FIELD

This invention relates to an ion spectrometer (such as an ion mobilityspectrometer) or a mass spectrometer particularly comprising asequential scanning mass analyzer, such as quadrupole mass filter,magnetic sector, multi-reflection time-of-flight and electrostatic trapmass analyzers.

BACKGROUND TO THE INVENTION

Modern continuous ion sources provide up to 10⁹ ions per second in thecase of Electrospray Ionisation (ESI), photoionisation (PI) andAtmospheric Pressure Chemical Ionisation (APCI) sources, up to 10¹⁰ ionsper second in the case of Electron Ionisation (EI) sources and up to10¹¹ ions per second in the case of Inductively Coupled Plasma (ICP) ionsources. Desirably, for the highest sensitivity and speed of analysis,mass analysis of ions from these ion sources would be achieved, withoutallowing ions to go to waste, on a continuous basis.

Sequential scanning mass analyzers comprise a mass filter, whichtransmits ions with only a narrow range of mass to charge ratios at agiven time. Hence, such analyzers waste any ions that are not of themass to charge ratio being transmitted, since these ions are lost.

This means that existing mass spectrometers using such analyzers operatewith a low duty cycle (typically between 0.1% and 10%) when performingquantitation analysis where multiple target compounds need to beanalysed. The use of continuous ion sources, such as electrosprayionisation sources, may mean that ions are wasted when the analyser dutycycle is so low.

Nevertheless, sequential scanning mass analyzers offer improvedlinearity and dynamic range over the alternatives. Hence, it isdesirable to improve the duty cycle of such mass spectrometers. Similarissues arise in relation to other types of ion spectrometer, such as ionmobility spectrometers or mass spectrometers, in which improvement inthe duty cycle may be especially beneficial.

SUMMARY OF THE INVENTION

Against this background, the present invention provides an ionspectrometer, comprising an ion source, arranged to generate ionscontinuously with a first range of mass to charge ratios; an ion trap,arranged to receive ions from the ion source along an axis, and to ejections with a second range of mass to charge ratios orthogonally to thataxis, the second range of mass to charge ratios being narrower than thefirst range of mass to charge ratios; a power supply, coupled to the ionsource and ion trap so as to provide a potential causing ions generatedby the ion source to continuously flow into the ion trap; and an ionanalyser, arranged to receive ions ejected from the ion trap andseparate the ions in accordance with at least one characteristic of theions.

The ion source produces a continuous beam of ions having a wide range ofmass to charge ratios and is coupled to the ion trap, so that ions fromthe source enter the ion trap in an uninterrupted stream. Ions of anarrow range of mass to charge ratios are ejected from the ion trapwhilst at the same time other ions received by the ion trap may becontinuously stored and held ready for future ejection. The ion trapthus acts as a combination of a continuously filled trap and a masspre-filter.

Hence, ions that are not required by the analyzer are not wasted, butcan instead be stored for ejection at a suitable instant. This can alsoimprove the dynamic range of the spectrometer, since the analyzer can befilled to its space charge capacity limit with more ions within theanalysis range. This also allows a detection limit to be reached in ashorter space of time.

Moreover, by ejecting ions orthogonally from the ion trap, the ion trapcan be closely coupled to the analyzer. This also reduces wastage ofions and improves speed. Together, these advantages allow more ions tobe usefully processed in a shorter space of time, such that thespectrometer can achieve a high duty cycle.

The present invention thereby provides effective improvement of the dutycycle close to the number of m/z windows of interest at a given moment.Typically, the duty cycle is improved to between 10% and 20%.

The term “continuous” as used herein can also relate to aquasi-continuous, intermittent or even pulsed ion current, underconditions in which ejection of ions from the trap is not tightlycorrelated with arrival of ions of a particular m/z ratio to the trap.

Advantageously, the power supply is further arranged to provide an RFpotential to the ion trap, so as to cause an electric field having aquasi-potential well in the dimension orthogonal to the axis along whichions with the second range of mass to charge ratios are ejected. As ionsare injected into the ion trap, their radial energy is not high enoughto escape from the quasi-potential well formed by the RF field. As theions then cool down during their stay within the trap, their radialenergy becomes even lower. Advantageously, the ions are caused to enterthe trap by a DC potential between the ion source and the ion trap. Inthis way, ions can be stored in the ion trap continuously, and at thesame time, be selectively ejected from the ion trap according to theirmass to charge ratio.

In the preferred embodiment, the spectrometer further comprises: ionoptics, arranged to receive ions ejected from the ion trap and cool theions. Preferably, the ion optics are arranged to cool the ions withoutsubstantial fragmentation. Moreover, the ion analyser may then bearranged to receive ions from the ion optics. Placing ion optics to coolthe ions between the ion trap and the ion analyser reduces the energy ofthe ions and allows their accurate analysis.

In a second aspect, the present invention may be found in an ionspectrometer, comprising: an ion source, arranged to generate ions witha first range of mass to charge ratios; an ion trap, arranged to receiveions from the ion source along an axis, and to eject ions with a secondrange of mass to charge ratios orthogonally to that axis, the secondrange of mass to charge ratios being narrower than the first range ofmass to charge ratios; ion optics, arranged to receive ions ejected fromthe ion trap and cool the ions without substantial fragmentation; and anion analyser, arranged to receive ions ejected from the ion optics andseparate the ions in accordance with at least one characteristic of theions.

The use of ion optics as a cooling guide makes this arrangement suitablefor simple spectrometry, with a high duty cycle.

The invention is particularly adapted for high space charge density, forinstance transferring more than 10⁸ charges per second through the iontrap. The purpose of this may be to improve the effective brightness ofthe ion source for the downstream analyser. The peak ion current withinions of the second range of mass to charge ratios ejected from the iontrap is at least: 5; 10; or 20 times higher than the average currentwithin the ions of the second range of mass to charge ratios received atthe ion trap from the ion source. Additionally or alternatively, theaverage ion current received by the mass filter may be at least 10 pA.

High space charge density may be achieved in a number of ways. Forexample, the length of rod electrodes in the traps or ion optics may bechosen to be substantial, such as hundreds of millimetres. Moreover, theinscribed radius of the rod electrodes may be large and the trappingfrequency may be high. The effect may further be improved when the ionsare scanned out quickly. Additionally, where the mass analyser theresolution of mass selection during scanning may be compromised.

A configuration similar to that of the present invention is shown incommonly assigned U.S. Pat. No. 7,157,698. However, this patent concernstandem mass spectrometry. In that case, ions are ejected from an iontrap to a collision cell, in which at some of the ions are fragmented.It is suggested there, that orthogonal mass-selective ejection in tandemmass spectrometry allows typically much higher ejection efficiencies,much higher scan rates, better control over ion population as well ashigher space charge capacity. However, it was not previously understoodthat these advantages would also be applicable for single massspectrometry. However, by ejecting ions from the ion trap to ion opticsfor cooling, similar advantages may be obtained for single massspectrometry as well, especially when a high space charge density isimplemented, as discussed above. In a further embodiment, the ion sourcemay be a continuous ion source and the ion trap receives ionscontinuously from the ion source along the axis.

A number of optional, preferable and advantageous features can beapplicable to either of these two aspects. Some of these are nowdiscussed below.

The invention is especially applicable to spectrometers comprising amass analyser, mass filter, ion mobility analyser or any combination ofthese. In some embodiments, the ion analyser comprises (or is) a massfilter or mass analyser. In preferred embodiments, the ion analysercomprises (or is) a sequential scanning mass filter, arranged to receiveions ejected from the ion trap and to transmit ions sequentiallyaccording to their mass to charge ratio. Additionally or alternatively,the ion analyser comprises an ion mobility analyser.

Preferably, the ion trap is further arranged to store the received ionsand to continue to store any received ions that are not ejected.

In the preferred embodiment, the ion trap comprises a power supply and aplurality of electrodes. The power supply may be arranged to supply tothe plurality of electrodes one or more of: a DC potential; an RFpotential; and an excitation potential. When the power supply appliesthe excitation potential to the plurality of electrodes, the powersupply may be further arranged such that the excitation potential causesions with the second range of mass to charge ratios to be ejected fromthe ion trap. Ejection of the ions by their axial excitation may alsoassist in allowing continuous filling of the ion trap at the same time.Only ions stored in the ion trap that are in resonance with the fieldgenerated by the excitation potential across the electrodes (preferablyrods) will acquire radial energy. This energy will grow until they getejected from the ion trap, overcoming the quasi-potential well generatedby the RF field. By changing the frequency of the excitation potential,ions of different mass to charge ratios may be ejected. Preferably, theion trap comprises a quadrupole ion trap.

In this way, the ions are ejected through the entire length of theelectrodes in a ‘ribbon’ beam. This results in an improved ion ejectionenergy, which is less dependent on space charge. Moreover, it allows anincrease in the space charge capacity of the ion trap withoutcompromising its performance, speed or efficiency of ejection. Such anarrangement is especially advantageous with the high space chargedensity embodiments discussed above.

Advantageously, the ion optics comprises a collision cooling guidearranged to receive ions from the ion trap. This may comprise a gas andbe arranged to cause the ions to collide with the gas so as to cool theions, and to eject the ions to the ion analyser. Beneficially, the iontrap and collision cooling guide share a common housing. Preferably, thecollision cooling guide comprises an RF ion guide. Ions are then stillefficiently cooled during their travel, thus being prepared forinjection into a mass analyser or ion mobility analyser. Hence, thepresent invention may represent a fusion of quadrupole and linear traptechnologies.

Preferably, the ion trap comprises an exit slit and the collisioncooling guide comprises an entrance slit. The exit slit of the ion trapmay then advantageously be located adjacent the entrance slit of thecollision cooling guide.

The ion optics (in the preferred embodiment, a collision cooling guide)are preferably arranged to receive ions from the ion trap along aprimary axis and to eject ions along a secondary axis, the secondaryaxis being substantially orthogonal to the primary axis. Optionally, thesecondary axis of the collision cooling guide may be parallel with theaxis of the ion trap.

Where the collision cooling guide is an RF ion guide, this RF ion guidemay comprise a power supply and a plurality of electrodes. Then, thepower supply may be arranged to supply DC potentials to the plurality ofelectrodes such that a potential well is generated along the primaryaxis. This causes the ions to be trapped in the collision cooling guidefor effective cooling.

According to another aspect, the present invention may be found in amethod of ion spectrometry, comprising: generating ions continuously inan ion source with a first range of mass to charge ratios; receivingions continuously from the ion source at an ion trap along an axis;ejecting ions with a second range of mass to charge ratios from the iontrap orthogonally to that axis, the second range of mass to chargeratios being narrower than the first range of mass to charge ratios;receiving ions ejected from the ion trap at an ion analyser.

Advantageously, the method further comprises storing ions received atthe ion trap along the axis, by providing an RF potential to the iontrap, so as to cause an electric field having a quasi-potential well inthe dimension orthogonal to the axis along which ions with the secondrange of mass to charge ratios are ejected.

In the preferred embodiment, the method further comprises: receivingions ejected from the ion trap at ion optics; cooling ions received atthe ion optics; and ejecting the cooled ions to the ion analyser.Preferably, the step of cooling ions is performed without substantialfragmentation of the ions.

In a yet further aspect, the invention may be found in a method of massspectrometry, comprising: generating ions in an ion source with a firstrange of mass to charge ratios; receiving ions from the ion source at anion trap along an axis; ejecting ions with a second range of mass tocharge ratios from the ion trap orthogonally to that axis, the secondrange of mass to charge ratios being narrower than the first range ofmass to charge ratios; receiving ions ejected from the ion trap at ionoptics; cooling ions received at the ion optics without substantialfragmentation; ejecting the cooled ions to an ion analyser; andreceiving ions ejected from the ion optics at the ion analyser.

A number of optional, preferable and advantageous features can beapplicable to either of these two method aspects. Some of these are nowdiscussed below.

In some embodiments, the ion analyser comprises (or is) a mass filter.Preferably, the ion analyser comprises (or is) a sequential scanningmass filter. Then, the method may further comprise ejecting ionssequentially according to their mass to charge ratio from the sequentialscanning mass filter. Additionally or alternatively, the ion analysercomprises (or is) an ion mobility analyser.

Optionally, the ion optics may comprise a collision cooling guide,comprising a gas. Then, the step of cooling the ions may comprisecausing the ions to collide with the gas so as to cool the ions. In someembodiments, the ion trap and collision cooling guide may share a commonhousing. Preferably, the collision cooling guide comprises an RF ionguide. More preferably, the ion trap comprises an exit slit and thecollision cooling guide comprises an entrance slit and the exit slit ofthe ion trap is located adjacent the entrance slit of the collisioncooling guide.

In the preferred embodiment, the step of receiving ions at the ionoptics from ion trap takes place along a primary axis and the step ofejecting ions from the ion optics takes place along a secondary axis.Advantageously, the secondary axis is orthogonal to the primary axis.Beneficially, the secondary axis along which the ions are ejected fromthe ion optics is parallel with the axis of the ion trap along whichions are received from the ion source.

In embodiments where the ion optics comprises an RF ion guide, the stepof receiving ions at the ion optics optionally comprises applying DCpotentials to a plurality of electrodes of the RF ion guide such that apotential well is generated along the primary axis.

In some embodiments, the peak ion current within ions of the secondrange of mass to charge ratios ejected from the ion trap is at least: 5;10; or 20 times higher than the average current within the ions of thesecond range of mass to charge ratios received at the ion trap from theion source. Additionally or alternatively, the average ion currentreceived by the ion analyser is at least 10 pA.

Preferably, the method also comprises: storing the ions received at theion trap; and continuing to store any ions received at the ion trap thatare not ejected from the ion trap. Optionally, the number of ions withinsecond range of mass to charge ratios is no more than 10% of the numberof ions within the first range of mass to charge ratios.

As used herein, ejection orthogonal to the axis of the ion trap is to beunderstood to mean ejection which comprises a range of ejection angleslargely centred upon an angle orthogonal to the axis of the ion trap.The axis of the ion trap is preferably straight, but may be curved. Inembodiments in which the axis of the ion trap is curved, ejection ateach point along the axis of the ion trap is largely centred upon anangle orthogonal to the local axis of the ion trap at that point. Theejection angle is largely centred upon an angle orthogonal to the iontrap but the presence of the potential between the ion source and theion trap which facilitates continuous filling of the ion trap mayintroduce an offset angle, for example.

A curved axis may provide the benefit of accelerated transfer of ionsfrom the cooling guide towards a quadrupole mass filter or other device.The offset angle might also appear as the result of any remaining axialenergy, especially at low gas pressure, or the focusing action of ionoptics during transfer.

It will also be understood that the present invention is not limited tothe specific combinations of features explicitly disclosed, but also anycombination of features that are described independently and which theskilled person could implement together.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be put into practice in various ways, one of whichwill now be described by way of example only and with reference to theaccompanying drawings in which:

FIG. 1 shows a schematic diagram illustrating a first embodiment of thepresent invention;

FIG. 2 shows a schematic diagram illustrating a second embodiment of thepresent invention;

FIG. 3 shows a top view of an embodiment of the present inventionaccording to the schematic diagrams of FIGS. 1 and 2;

FIG. 4 shows a side view of the embodiment of FIG. 3;

FIG. 5 shows the side view of FIG. 4 for an alternative embodiment ofthe invention;

FIG. 6 shows a plot of potential against distance for the embodiment ofFIGS. 3, 4 and 5, operating in a high duty cycle mode.

SPECIFIC DESCRIPTION OF A PREFERRED EMBODIMENT

Referring first to FIG. 1, there is shown a schematic diagramillustrating a first embodiment of the present invention.

A mass spectrometer 10 comprises an ion source 20; an ion trap 40; and asequential scanning mass analyser 60. The ion source 20 generates ions30 continuously with a first range of mass to charge ratios. The iontrap 40 receives ions 30 continuously with the first range of mass tocharge ratios and orthogonally ejects ions 50 with a second range ofmass to charge ratios. The second range of mass to charge ratios isnarrower than the first range of mass to charge ratios. The ions 50 arethen received by the sequential scanning mass analyser 60 for analysis.

Referring next to FIG. 2, there is shown a schematic diagramillustrating a second embodiment of the present invention.

A mass spectrometer 10′ comprises an ion source 20; an ion trap 40; acooling guide 45 and a sequential scanning mass analyser 60. The ionsource 20 generates ions 30 with a first range of mass to charge ratios.The ion trap 40 receives ions 30 with the first range of mass to chargeratios and orthogonally ejects ions 50 with a second range of mass tocharge ratios to the cooling guide 45. The second range of mass tocharge ratios is narrower than the first range of mass to charge ratios.The cooling guide 45 cools the received ions 50, without substantialfragmentation. The energy of the ions is typically reduced to a few eVor even to less than 1 eV. Cooled ions 48 are ejected axially to thesequential scanning mass analyser 60 for analysis.

Referring now to FIG. 3, there is shown a top view of an embodiment ofthe present invention according to the schematic diagrams of FIG. 1 andFIG. 2. Where components identical to those in FIG. 1 are shown, thesame reference numbers are used.

Ion source 20 ejects ions 30 with a first range of mass to chargeratios. Quadrupole ion trap 100 receives ions 30, and orthogonallyejects ions 50, with a second range of mass to charge ratios, tocollision cooling guide 110. Cooled ions 120 are ejected from collisioncooling guide 110 to mass analyser 60.

Ions 30 continuously enter quadrupole ion trap 100 from ion source 20.This is effected by a DC potential gradient between the ion source 20and the entrance to the ion trap 100. Quadrupole ion trap 100 isgas-filled and comprises rods 105. The potentials on the rods cause ions50, with a second range of mass to charge ratios, to be ejected tocollision cooling guide 110. Collision cooling guide 110 comprises rods115. The potentials on the rods cause ions 120 to be axially ejected tomass analyser 60.

FIG. 4, which shows a side view of the embodiment of FIG. 2, providesmore information on how the potentials on rods 105 and rods 115 areconfigured.

Quadrupole ion trap 100 acts as an excitation guide. Rods 105 areprovided with DC and RF potentials, which cause an electric trappingfield that holds received ions 30. This causes a quasi-potential barrierseparating ions 30 from the adjacent collisional cooling guide 110.

As ions are injected into the quadrupole ion trap 100, their radialenergy is not high enough to escape from the quasi-potential barrierformed by the RF potentials. As the ions cool down, their radial energybecomes even lower. Thus, the ions become trapped in the ion trap 100.

Rods 105 are also provided with an excitation potential. The excitationpotential causes rapid low-quality resonant excitation such that onlyions with a certain mass to charge ratio (m/z) sub-range acquire radialenergy sufficient to overcome the quasi-potential barrier and arethereby ejected orthogonally from the trap through an exit slit. Bychanging the frequency of excitation, ions of different m/z may beejected. This may operate in a similar way to the quadrupole ion trapdescribed in commonly assigned patent, U.S. Pat. No. 7,157,698. However,due to the application of DC potentials between the ion source 20 andion trap 100 as explained above, ions continue to enter the trap evenduring this ejection process. This continuous injection advantageouslyoperates in combination with orthogonal ejection of a selected subset ofthe ions.

This is illustrated in FIG. 6, which shows a plot of potential againstdistance for the embodiment of FIGS. 3, 4 and 5, operating in a highduty cycle mode. Curve 200 shows the DC potentials in the directiontransverse to rods 105. Curve 210 shows the quasi-potential barrier.Typically, only 5%-10% of the total mass range is orthogonally ejectedin this way.

Ions remaining in the quadrupole ion trap 100 are cooled and storeduntil their turn or until all ions are purged, for instance by reducingthe RF potential, or by removal of the DC barrier in the cooling guide110.

The RF-pseudo-potential is proportional to the square of the RFamplitude. If the amplitude is increased twofold, the RFpseudo-potential well appears to be four times deeper, when the RFfrequency is fixed. Thus, the AC amplitude should be increased by afactor of four and the AC frequency by factor of two.

Returning again to FIG. 3, the collisional cooling guide 110 isgas-filled and has a large width and comprises rods 115. Ions enter theguide 110 from the quadrupole ion trap 100 in the direction transverseto rods 115. Rods 115 are provided with a retarding DC field in thistransverse direction to allow sufficient length of travel of the ions onentry to the guide 110. The collisional cooling guide 110 is providedwith an exit 130.

Collisions with gas over this substantial length dampen the ion energyand they relax to the bottom of a DC potential well 220, which isillustrated in FIG. 6. This well is aligned to the exit from the coolingguide 130. This allows ions 120 to leave this guide into mass analyser60.

In such an embodiment, the total time for excitation and transferbetween source 20 and mass analyser 60 is not more than few ms. Thedwell time in mass analyser is also less than 1 to 3 ms. The gaspressure in the collision cooling guide 110 is between 0.001 and 0.01mbar. The pressure multiplied by length is between 0.03 and 0.5 mbar*mm

The gas in the collisional cooling guide 110 is preferably one or moreof: helium, nitrogen, argon. Rods 105 and rod 115 are preferably roundin section or generally round in section with hyperbolic profile towardsthe axis and a diameter of between 1.5 to 3 mm. The distance between thecentres of adjacent rods 105 in quadrupole ion trap 100 is between 1.3to 2 times their diameter. The distance between the centres of adjacentrods 105 in collisional cooling guide 110 is between 1.5 to 3 times thedistance between adjacent rods in quadrupole ion trap 100. Some or allof the six pairs of rods shown in FIG. 3 may be sectioned.

The mass resolving power of the quadrupole ion trap 100 is between 10and 20. This is lower than that of the mass analyser 60. The total cycletime for covering the entire mass range is between 30 to 50 ms. Hence,space charge does not affect the excitation process beyond usability.

The lengths of the excitation guide 100 and collisional cooling guide110 are 30 to 100 mm. This provides a balance between the desire tomaximize space-charge capacity whilst achieving improved speed of iontransfer to the mass analyser.

Referring now to FIG. 5, there is shown the side view of FIG. 4 for analternative embodiment of the invention. Vanes are also introduced intothe cooling guide. For this embodiment, the electrode voltages in fulltransition mode may be as follows.

E1 E1a E2 E2a E3 E3a E4 E4a E5 E5a E6 E6a RF, V 150 −150 −150 150 700−700 −700 700 700 −700 −700 700 DC, V 13 4 4 13 3.6 3.6 −3 −3 3 3 6 6

Whilst a specific embodiment has been described, the skilled person maycontemplate various modifications and substitutions. For instance, axialmovement of ions along the cooling guide 110 and transfer into massfilter could be accelerated by any known means of creating axial field,for example, resistive rods or vanes.

The embodiment described above relates to the use of a sequentialscanning mass analyser, but it will be appreciated that other types ofmass filters or mass analysers may be used. The invention may also beused in conjunction with an ion mobility analyser, in which case thiswould replace the sequential scanning mass analyser 60 in thearrangement discussed above. Most preferable designs of ion mobilityanalyser are described in US-2010/243883, GB-2486584, GB-2382919.

Further detection systems may also be provided. These may be used in thecombination where the analyser part of the arrangement (that is,downstream from the ion trap 40 or cooling guide 45) comprises: an ionmobility analyzer followed by a time-of-flight mass analyser; a massfilter followed by a time-of-flight mass analyser; an ion mobilityanalyzer or a mass filter followed by an ion trap or a fragmentationcell followed by an analyser, such as a followed by a time-of-flightmass analyser or orbital trapping mass analyser, such as that marketedby Thermo Fisher Scientific under the brand name Orbitrap™; and othersimilar combinations.

In some configurations, resonant excitation in the ion trap 40 may beachieved by a first RF potential and an auxiliary RF potential. Adding asecond (or more than one) RF potential may allow the simultaneousselection (by resonant excitation) of ions of multiple masses or massranges, based on the RF potentials applied.

The system could have also other preceding separations which may changea composition of the ion current coming into it, such as mass analysers(for instance, quadrupole, time-of-flight, magnetic sector, etc.) or ionmobility analysers of any type (e.g. field-asymmetric, differential,drift tube, running wave(s), rotating-field, gas flow assisted, etc.).For example, the invention can be used in a tandem quadrupole massspectrometer, located upstream from a first quadrupole analyser or acollision cell such as a travelling wave (T-Wave) collision cell. It mayfurther be appreciated that the invention may be applicable to aquadrupole time-of-flight (QTOF) mass spectrometer, for instanceupstream or in place of a first quadrupole mass analyser. Then, thedownstream devices, which may comprise a quadrupole (that is known to bea sequential scanning mass filter) or a travelling wave collision cell(which can be used as a sequentially scanning ion mobility analyzer)acting as real sequential scanning devices. Alternatively, atime-of-flight mass analyser may be located downstream of the invention,which may be so fast so as to have similar properties as a quadrupolemass filter. The invention could also be used between a MALDI source andan ion mobility cell of an ion instrument, such as described inWO-2010/085720 and in particular as shown in FIG. 1 of this document,especially when the MALDI “shot” frequency is high compared with themass ejection rate of the ion trap of our description. This might beunderstood as a continuously firing laser, as suggested above.Ion-molecule and ion-ion reactions, collisions with gas, irradiation byphotons could also be used, for example, to affect a composition of theion current.

What is claimed is:
 1. An ion spectrometer, comprising: an ion source,arranged to generate ions continuously with a first range of mass tocharge ratios; a first ion trap, arranged to receive ions from the ionsource along an axis, and to eject ions with a second range of mass tocharge ratios orthogonally to that axis, the second range of mass tocharge ratios being narrower than the first range of mass to chargeratios; a power supply, coupled to the ion source and the first ion trapso as to provide a potential causing ions generated by the ion source tocontinuously flow into the first ion trap and to provide an excitationpotential to the first ion trap to eject the ions with the second rangeof mass to charge ratios while at the same time other ions are receivedby the first ion trap and stored for future ejection; a mass selectorarranged to receive ions ejected from the first ion trap and configuredto selectively transmit a subset of the ejected ions based on at leastone property of the ions; at least one of a second ion trap and acollision cell for receiving the ions selectively transmitted by themass selector; and an analyzer arranged to receive ions from the atleast one of the second ion trap and the collision cell, for massanalyzing the received ions.
 2. The ion spectrometer of claim 1, whereinthe mass selector comprises a mass filter.
 3. The ion spectrometer ofclaim 1, wherein the mass selector comprises an ion mobility separator.4. The ion spectrometer of claim 1, wherein the analyzer comprises atime-of-flight mass analyzer.
 5. The ion spectrometer of claim 1,wherein the analyzer comprises an orbital trapping mass analyzer.
 6. Amass spectrometry method, comprising: generating ions continuously in anion source with a first range of mass to charge ratios; receiving ionsfrom the ion source at a first ion trap along an axis; ejecting ionswith a second range of mass to charge ratios from the first ion traporthogonally to that axis, the second range of mass to charge ratiosbeing narrower than the first range of mass to charge ratios;concurrently with the ejecting step, receiving additional ions from theion source at the first ion trap; selecting a subset of the ejected ionsfor analysis based on at least one property; directing the selected ionsto at least one of a second ion trap and a collision cell; and massanalyzing the selected ions, or ions derived therefrom, at a massanalyzer positioned to received ions from the second ion trap orcollision cell.
 7. The method of claim 6, wherein the step of selectinga subset of the ejected ions comprises filtering ions in accordance withtheir mass to charge ratio.
 8. The method of claim 6, wherein the stepof selecting a subset of the ejected ions comprises separating ionsaccording to their ion mobility.
 9. The method of claim 6, wherein thestep of directing the selected ions to at least one of an ion trap and acollision cell comprises fragmenting the ions in the collision cell toproduce product ions.
 10. The method of claim 6, wherein the number ofions within the second range of mass to charge ratios is no more than10% of the number of ions within the first range of mass to chargeratios.
 11. The method of claim 6, wherein the first ion trap comprisesa quadrupole ion trap and wherein the step of ejecting ions with asecond range of mass to charge ratios from the first ion trap comprisesapplying an excitation potential to a plurality of electrodes of thequadrupole ion trap, thereby causing ions with the second range of massto charge ratios to be ejected from the first ion trap.
 12. The methodof claim 6, wherein the step of mass analyzing is performed by atime-of-flight mass analyzer.
 13. The method of claim 6, wherein thestep of mass analyzing is performed at an orbital trapping massanalyzer.